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Re: Pulsed Tube Coils
> Subscriber: wb8jkr-at-juno-dot-com Sat Jan 11 10:42:29 1997
> Date: Fri, 10 Jan 1997 06:52:28 EST
> From: Mark S Graalman <wb8jkr-at-juno-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Pulsed Tube Coils
>
>
> On Thu, 9 Jan 1997 22:39:01 -0700 Tesla List <tesla-at-poodle.pupman-dot-com>
> writes:
> >> Subject: Re: Pulsed Tube Coils
> >> Subject: Re: Pulsed Tube Coils
> >
> >Subscriber: rwstephens-at-headwaters-dot-com Thu Jan 9 22:35:31 1997
> >Date: Thu, 9 Jan 1997 22:07:41 -0500
> >From: "Robert W. Stephens" <rwstephens-at-headwaters-dot-com>
> >To: tesla-at-pupman-dot-com
> >Subject: Re: Pulsed Tube Coils
> >
> >> Date: Mon, 6 Jan 1997 22:43:07 -0700
> >> From: Tesla List <tesla-at-poodle.pupman-dot-com>
> >> To: Tesla-list-subscribers-at-poodle.pupman-dot-com
> <SNIP>
> >I have had plans to attempt voice modulation of the output lightning
> >storm from a disruptive discharge TC for some time now and know
> >exactly how I'm going to attempt it once I finally 'get a round
> >tuit'. For now I'll only share that my proposed technique recognizes
> >Niquist's Theorem for audio sampling rate and will necessarily employ
> >a rapid rotary break to sample the audio spectrum where most energy
> >in human speech is concentrated. This should produce an amazing stage
> >effect.
> >If 'Man or Astroman' can receive these waves I'd like to hear from
> >you guys. I might be able to build a TC for you that will not only
> >allow you to communicate with your home planet, but do a neat stage
> >show here on earth as well!
> >
> >If anyone ever picks up one of my modulated TC experiments and can
> >prove it I promise to QSL!
> >
> >rwstephens
> >
>
> Robert,
>
> Thats pretty much what I was originally talking about,
> using a transistor switch in the cathode return of the
> tube turned on and off at a audio rate, with a filtered
> DC plate supply. Anyway, I just wish I had time to play
> with half the stuff I think about.<G> Plate modulation
> would be a heck of a nice way to go, but I don't have
> 500 or so watts of audio available or a suitable modulation transformer.
> Maybe a good way for a high power tube coil to be modulated would be to
> do it at
> the screen with a tetrode like a 4-400 or a 813.
>
>
> Mark Graalman
>
Mark,
If you want quality modulation with low audio driving power, give
screen grid modulation using a modest audio output transformer
backwards driven from an audio amplifier a try. With your idea of
transistor(zzzzzzz) in parallel in the cathode circuit you are still
going to have to create what is effectively that 500 watt or whatever
audio power amp, only this time it will have to be class-A (ouch).
The 813 tube sounds like a good one to start with. It is really easy
to melt the screen grid on a tetrode (you really should install RF bypassed
milliamper meters on all your grids and cathode circuit when
breadboarding something like this and watch those grid currents). I melted the
screen in a NEW 4CX10,000D Eimac tube because I didn't bother with enough
meters while breadboarding a cluged together circuit and I'm not very pleased
with myself as a result. : ( I also DON'T LIKE TETRODES much now (go
figure). At least in a toob like the 813 you can SEE if you are thermally
stressing the elements, often early enough to shut down power and save the tube.
Oh, another thing about tetrodes. NEVER allow the screen grid to
have accelerating voltage on it with plate voltage absent. The
screen will look like a plate to the electron stream and turn to
vapor really quickly. If your screen supply is just a resistive tap from
the plate supply that is an acceptable design practice as both
elements will be powered up simultaneously. If there are separate power
supplies, relay logic or other protection circuits are necessary to
prevent the application of screen potential before there is nearly
full plate voltage. I know this rule well and it was not the cause of my own
meltdown however.
BTW, you might try half a neon sign transformer as a plate modulation
transformer for a low powered tube coil. Insulate the case and run
it to your B+ supply. Run the highest impedance output tap you have from
your audio amp into the 120 volt winding on the neon. If you have a 70 volt line
tap that would be terrific. Make sure one side of your audio amp
output is a good ground. If the B+ arcs internally in the neon from
the core to the primary winding you dont want it taking out your
amplifier. I would always feel more comfortable doing such
experiments with a vacuum tube hi fi amplifier with its own isolating
output transformer built in. I haven't tried a neon and suspect the
results will be rather poor but mention it in case you have one at
hand. I have actually tried this trick with a HV plate transformer and it worked
quite well. Put the HV winding in series with the filtered DC plate supply.
Be sure to run a protective spark gap across the HV winding if you can, and at
the very least a small value ceramic disc or mica cap good for maybe 5 kV at
0.001to 0.01 mfd to keep RF out of the transformer.
Good luck.
rwstephens